An optical waveguide device that uses an electro-optic crystal such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO2), is formed by forming a metal film of titanium (Ti) or the like on a part of a crystal substrate, to be thermally defused, or to be patterned, after which it is proton exchanged or the like in benzoic acid, to form an optical waveguide, and thereafter an electrode is provided in the vicinity of the optical waveguide. As such an optical waveguide device that uses an electro-optic crystal, there is known for example an optical modulator as illustrated in FIG. 1.
In FIG. 1, an optical waveguide formed on a substrate 100 comprises; an input waveguide 121, an optical branching section 122, a pair of branching waveguides 123 and 124, an optical multiplexing section 125, and an output waveguide 126. A signal electrode 131 and an earth electrode 132 are provided on the pair of branching waveguides 123 and 124, to form a coplanar electrode. In the case where a Z-cut substrate is used, in order to use the refractive index variation due to the electric field in the Z direction, the signal electrode 131 and the earth electrode 132 are arranged directly above the optical waveguides. More specifically, the electrodes are patterned with the signal electrode 131 on the branching waveguide 123, and the earth electrode 132 on the branching waveguide 124. Here in order to prevent the light that is propagated through the branching waveguides 123 and 124 from being absorbed by the signal electrode 131 and the earth electrode 132, a buffer layer (not illustrated in the figure) is provided between the substrate 100, and the signal electrode 131 and the earth electrode 132. For the buffer layer, an oxide silicon (SiO2) or the like of 0.2 to 2 μm thickness is used.
In the case where such an optical modulator is driven at high speed, the output end P4 of the signal electrode 131 is connected to the earth electrode 132 via a resistance (not illustrated in the figure) to make a travelling wave electrode, and a microwave electric signal RF is applied from the input end P3 of the signal electrode 131. At this time, due to the electric field generated between the signal electrode 131 and the earth electrode 132, the refractive indices of the branching waveguides 123 and 124 respectively change as +na and −nb, so that the phase difference of the light propagated on the branching waveguides 123 and 124 changes. Therefore, a light Lin input to the input port P1 is intensity modulated by Mach-Zehnder (MZ) interferometer, and modulation light Lout is output from the output port P2. The range INT illustrated by the arrow in the figure, represents the part in which light and electric field interact, and in the following description, this is referred to as the “interaction portion”. Furthermore, the longitudinal direction of the substrate 10 (the propagation direction of the light in the interaction portion INT) is the x direction, and the direction perpendicular to the x direction is the y direction. By changing the cross-section shape of the signal electrode 131 to control the effective refractive index of the microwave electric signal RF, and by matching propagation speeds of the light and the microwave electric signal with each other, high speed optical response characteristics can be obtained.
Furthermore, due to the variety of recent optical modulation formats (for example multi-valued modulation format, optical polarization division multiplexing format, and the like), there are many cases where signals corresponding to a desired optical modulation format are generated, by combining a number of conventional optical modulators such as illustrated in FIG. 1 (for example, refer to Japanese Laid-open Patent Publication No. 2008-122786).
In the above described configuration where a plurality of optical modulators are combined, in order to reduce the size of the overall optical modulator, it is effective to integrate respective optical modulators on a single chip (substrate). In the following description, individual optical modulators integrated on a single chip is referred to as “an optical modulation section”.
More specifically, the optical modulator illustrated in FIG. 2 is a configuration example of where two optical modulation sections 120A and 120B are arranged in parallel on a single substrate 100. The optical modulation sections 120A and 120B, similarly to the configuration illustrated in FIG. 1, each have an MZ type optical waveguide, a signal electrode, and an earth electrode. Furthermore, as the optical waveguide for inputting light to the respective optical modulation sections 120A and 120B, an input waveguide 111 connected to one input port P1 which is one end face of the substrate 100, is branched into two curved waveguides 113A and 113B by an optical branching section 112, and the respective curved waveguides 113A and 113B are connected to input waveguides 121A and 121B of the respective optical modulation sections.
In the above described configuration, considering to apply electric signals RFA and RFB from the outside to signal electrodes 131A and 131B of the respective optical modulation sections 120A and 120B, electrode input terminals are provided in a package (not illustrated in the figure) for accommodating the substrate 100. If electrode input terminals respectively corresponding to the optical modulation sections 120A and 120B are placed side by side on the side face on one side of the package, mounting of the substrate 100 can be facilitated, and the mounting footprint made small. In this case, for the respective signal electrodes 131A and 131B on the substrate 100, electrode pads formed near each of input ends P3A and P3B are arranged side by side on one side (the lower side in the figure) of the opposite side faces of the substrate 100.
In the electrode pads near the respective input ends P3A and P3B, in order to connect to the outside (the electrode input terminals of the package) with wire bonding or the like, it is necessary to have a certain amount of spacing. Therefore, as illustrated at the top of FIG. 2, the location of the start points of the interaction portions INTA and INTB in the respective optical modulation sections 120A and 120B are displaced by the spacing dxE of the input ends P3A and P3B of the respective signal electrodes 131A and 131B. As a result, the length of the interaction portion INTB of one optical modulation section 120B becomes shorter than the length of the interaction portion INTA of the other optical modulation section 120A, and hence the drive voltage on the optical modulation section 120B side rises.
Instead of making the interaction portion INTB of the optical modulation section 120B short, then as illustrated at the center of FIG. 2, the size of the substrate 100 is extended in the lengthwise direction (x direction), and the location of the interaction portions INTA and INTB of the respective optical modulation sections 120A and 120B are moved in the x direction corresponding to the spacing dxE of the input ends P3A and P3B of the respective signal electrodes 131A and 131B, so that the length of the respective interaction portions INTA and INTB can be made equal. Here, the output ends P4A and P4B of the respective signal electrodes 131A and 131B are arranged side by side on the other face (the upper face in the figure) of the opposite side faces of the substrate 100. However this configuration incurs a lengthening of the size of the package and enlargement of the optical modulator. Furthermore, the number of chips (substrates 100) that can be cut from a single chip is reduced, so that manufacturing costs are increased.
In order to arrange the electrode pads of the respective signal electrodes 131A and 131B side by side on one side of the substrate 100 without shortening the interaction portion or extending the substrate size in the longitudinal direction, then as illustrated at the bottom of FIG. 2, it is necessary to shorten the length dx0 from the end face (hereunder referred to as the input end face) where the input port P1 of the substrate 100 is located, to the optical branching sections 122A and 122B of the respective optical modulation sections 120A and 120B. However, in this case, the radii of curvature of the respective curved waveguides 113A and 113B becomes small. Therefore the bend loss (radiation loss) in the curved waveguides 113A and 113B is increased, and the input light intensity of the respective optical modulation sections 120A and 120B is reduced.